Also listen to the engine for a "sewing machine" sound coming from the timing chain cover. Don't confuse the loud fuel injectors with this cause some do. It may be hard to do since this is a van, but if you can identify a noise coming from this area, it is a good indication that the plastic timing chain guides and tensioners have failed. They are a common problem on these engines and when the plastic comes off, it can get caught on the oil pickup screen and cause low oil pressure at times.
But like henry10 suggested, start cheap and easy and work your way from there.